Mpg123 is a fast, free and portable MPEG audio player and decoder library mainly for Unix (but also ported to other platforms). It supports MPEG 1.0/2.0/2.5 layers 1, 2, and 3, with CD quality playback even on relatively slow CPUs.

LAME (LAME Ain't an MP3 Encoder) is a research project for learning about and improving MP3 encoding technology. LAME includes an MP3 encoding library, a simple frontend application, and other tools for sound analysis, as well as convenience tools. Please note that any commercial use (including distributing the LAME encoding engine in a free encoder) may require a patent license from Thomson Multimedia.

Ampache is a Web-based audio file manager. It allows you to view, edit, and play your audio files via the Web. It has support for playlists, artist and album views, album art, random play, playback via HTTP with on-the-fly transcoding and downsampling, vote based playback, playback via MPD and Icecast, an integrated Flash player, and per-user themes and song play tracking. You can also link multiple Ampache servers together using XML-RPC. Ampache supports gettext translations and has full translations into many languages.

Audacity is a cross-platform multitrack audio
editor. It allows you to record sounds directly or
to import Ogg, WAV, AIFF, AU, IRCAM, or MP3 files.
It features a few simple effects, all of the
editing features you should need, and unlimited
undo. The audio I/O uses PortAudio, which fully
supports OSS, Mac OS X CoreAudio, and Windows
WMME, and can be compiled with support for ALSA
and Jack.

GeeXboX is a standalone media player Linux
distribution, similar to MoviX. It's a small
bootable CD that allows you to play your favorite
video (DivX, XviD, H.264, MPEG 1/2, VCD, DVD,
OggMedia, Windows Media, RealMedia, etc.) and
audio (MP3, Audio CD, Ogg Vorbis, FLAC, MPC, etc.) files. It
also supports networking, and is able to play media
from Windows/Samba share, NFS, UPnP A/V Media Servers, RTP/RTSP servers, or SHOUTcast. It supports TV-out, TV tuners, DVB cards, and WiFi cards. It is based on MPlayer, and can be used on any x86, x86_64, or PowerPC computer. It's easy to modify the source to build your own GeeXboX or use an alternative
boot method.

abcde is a frontend to cdparanoia, wget, cd-discid, id3, and your favorite Ogg Vorbis (the default), MP3, FLAC, Ogg Speex, or MPP (Musepack) encoder. It grabs an entire CD and converts each track to the desired format, then comments or ID3-tags each file, all with one command. It supports multiple output in a single CD read, the creation of a single track from a CD, resume operation, CD concatenation, volume normalization, gapless encoding (with LAME), parallelization, SMP, proxies, customizable filename organization and munging, playlist generation, distributed encoding via distmp3, and more.

Beets is a media library management system designed for obsessive-compulsive music geeks. It catalogs your collection, automatically improving its metadata as it goes using the MusicBrainz database. It also downloads cover art for albums it imports. Then it provides a bouquet of tools for manipulating and accessing your music. Beets also includes a music player that speaks the MPD protocol, so you can play music in your beets library using any MPD client.

Zina is a graphical interface to your MP3 collection, a personal jukebox, a Podcaster, and an MP3 streamer. It includes an optional Flash player, a tag editor, and can search the Web for album art and song lyrics. It can run alone, be embedded into an existing Web site, or be used as a module in Drupal, Joomla, and Wordpress.